We are studying the mechanism of genetic resistance to the leukemogenic and immunosuppressive effects of Friend leukemia virus. It appears that marrow dependent (M) cells, depleted by treatment of mice with 89Sr, regulate the numbers and functions of thymus dependent suppressor cells which are the targets of the virus for immunosuppression. In addition, M cells function as natural killer cells and as regulators of other types of suppressor cells. We shall try to determine if the M cells destroy the virus transformed cells. We are developing a treatment protocol for leukemia which involves irradiation and marrow cell grafting, employing donors genetically resistant to the leukemia. We will try to devise ways to prevent graft-versus-host disease, which is the major complication of marrow grafting between allogeneic donors and hosts.